The company reported a worse than expected fall in its key organic service revenue metric, down by 1.4% in the second quarter.
The organic metric decline is the first for Vodafone since 2010 and has been attributed to a slowdown in its southern European business, which declined by 11.3%.
Customers in southern Europe making fewer calls, the weakening of currencies in major markets and a slowdown of growth in emerging markets, including India and South Africa, is deemed to have affected the company's outlook.
Vodafone operations in north and central Europe recorded 0.7% growth and its operations in the rest of the world grew by 4.1%.
Vodafone's total revenues for the fiscal first half declined by 7.4% from the previous year to £21.78 billion, while adjusted operating profit improved 2.2% to £6.17 billion, largely due to a strong contribution from joint venture Verizon Wireless.
The company said it expects free cash flow for the full year to be in the lower half of its guidance range
